Apoptosis or Programmed Cell Death is an important biological process that is required to maintain the integrity and homeostasis of multicellular organisms. Apoptosis is initiated by extracellular or intracellular signals in which a complex machinery is activated to start a cascade of events that ultimately leads to the degradation of nuclear DNA and a dismantling of the cell. Apoptosis is a tightly regulated and conserved mode of cell death that does not result in injury to adjacent cells. However, necrosis is a catastrophic, unregulated mode of cell death that is characterized by an invasion of inflammatory cells and injury to adjacent tissue. Abnormal or unregulated apoptosis is believed to occur in a variety of disease states. For example, elevated apoptosis is believed to play a major role in ischemia-reperfusion injury (i.e., myocardial infarction and stroke), neurodegeneration (Parkinson's Disease, Alzheimer's Disease, ALS), sepsis, and diabetic cardiomyopathy. The inability of a cell to undergo apoptosis is believed to play a major role in the resistance of tumor cells toward radiation, chemotherapy, and immunotherapy. Therefore, the development of a noninvasive imaging procedure that can measure apoptosis is of importance to many institutes within the National Institutes of Health, and is consistent with the research mission of the National Institute of Biomedical Imaging and Bioengineering. The current method for imaging apoptosis uses radiolabeled analogs of Annexin V. Annexin V is a protein that binds with high affinity to phosphatidyl serine residues that are exposed as part of membrane inversion that occurs during apoptosis. However, since membrane inversion also occurs during necrosis, imaging studies using radiolabeled Annexin V measure both apoptosis and necrosis. An alternative strategy for measuring apoptosis is to determine the level ofcaspase-3 activity in the cell. Caspase-3 is an "executioner" caspase and is released from an inactive zymogen (procaspase-3) late in the process of apoptosis. Furthermore, evidence suggests that inhibitors of caspase-3 may provide an effective method for preventing cellular death that occurs in diseases characterized by an increase in apoptosis (i.e., ischemia-reperfusion injury, neurodegeneration). Therefore, a noninvasive imaging procedure that can quantify caspase-3 activity would be useful both in the study of apoptosis in a wide variety of clinical conditions, but would also be useful in assessing the role of caspase-3 inhibitors in preventing apoptotic cellular death. The goal of this research project is to develop fluorine-18 labeled imaging agents possessing a high affinity and selectivity for caspase-3. The design of the imaging agents is based on a number of small molecule inhibitors of caspase-3 that have appeared in the literature over the past year. Once a suitable 18F-labeled caspase-3 radiotracer has been identified from biodistribution and microPET imaging studies, a series of comparison studies between a caspase-3 radiotracer and radiolabeled Annexin V will be conducted in order to determine which agent provides an accurate measurement of apoptosis. If successful, the research described in this research proposal will provide a PET-based imaging procedure for measuring apoptosis that will be of tremendous value to the PET research community.